Methods for increasing the fatigue life, of crankshafts

Abstract

The invention relates to a method for increasing the bending strength and the endurance limit of crankshafts by locally restricted hammering in areas of high stress, such as grooves, the mouths of bores and cross-sectional transition zones, in which method pressure impulse machines or beating devices comprising heating tools are employed. The pressure impulse machines or beating devices only execute a relative displacement of the beating tool against the surface of the crankshaft segment to be processed when the compressive stress is introduced between the heating tool and said surface of the crankshaft segment to be processed. The invention also relates to a device for increasing the endurance limit of crankshafts.

1. A method for increasing the fatigue life, in particular the bending fatigue life and the torsional fatigue life of crankshafts, in particular of large crankshafts, by local hammering of highly loaded areas, such as grooves, hole mouths and cross-sectional junctions, by means of pulsed-pressure machines or striking apparatuses which introduce intrinsic compressive stresses into the crankshaft via striking tools, characterized in that the pulsed-pressure apparatuses or striking machines carry out only a relative movement on a plane at right angles to the surface of the crankshaft segment at the time at which the compressive stress is introduced between the striking tool and the crankshaft segment to be processed, with the crankshaft being rotated continuously during the processing, and in that the time during which the striking tool is acting and the striking pressures are chosen such that, when the striking tool strikes the crankshaft segment to be processed while the intrinsic compressive stresses are being introduced, the rotational movement of the crankshaft is necessarily stopped. 
     
     
       2. The method as claimed in  claim 1 , characterized in that the striking frequency of the striking tool is between 0.1 and 20 Hz. 
     
     
       3. The method as claimed in  claim 2 , characterized in that the striking frequency of the striking tool is between 1 and 10 Hz. 
     
     
       4. The method as claimed in  claim 3 , characterized in that the striking frequency of the striking tool is between 3 and 6 Hz. 
     
     
       5. The method as claimed in  claim 1 , characterized in that the striking pressures of the striking tool are between 10 and 300 bar. 
     
     
       6. The method as claimed in  claim 5 , characterized in that the striking pressures of the striking tool are between 30 and 130 bar. 
     
     
       7. The method as claimed in  claim 6 , characterized in that the striking pressures of the striking tool are between 50 and 110 bar. 
     
     
       8. The method as claimed in one of  claims 1  to  7 , characterized in that the temperature in the region of the crankshaft segment to be processed is less than 65° C. 
     
     
       9. The method as claimed in  claim 8 , characterized in that the temperature in the region of the crankshaft segment to be processed is between 12 and 25° C. 
     
     
       10. The method as claimed in  claim 1 , characterized in that the intrinsic compressive stresses are introduced by means of the striking tools on crankshafts which have already previously been processed by a method for increasing the fatigue-life characteristics. 
     
     
       11. The method as claimed in  claim 10 , characterized in that the intrinsic compressive stresses are introduced by the striking tools after induction hardening of the crankshaft. 
     
     
       12. The method as claimed in  claim 1 , characterized in that, once the intrinsic compressive stresses have been introduced by the striking tools, the intrinsic compressive stresses close to the surface are reduced by machining away the surface of the crankshaft segment to be processed. 
     
     
       13. The method as claimed in  claim 12 , characterized in that up to 3 mm of the surface of the processed crankshaft segment is removed. 
     
     
       14. The method as claimed in  claim 13 , characterized in that between 0.3 and 2 mm of the surface of the processed crankshaft segment is removed. 
     
     
       15. The method as claimed in  claim 12 , characterized in that the removal is carried out by grinding, turning or milling. 
     
     
       16. The method as claimed in  claim 1 , characterized in that, in one refinement of the crankshaft segment to be processed, and which is in the form of a catenary, the continuous junction radii which are in the form of an initial contour are compressed by the introduction of the intrinsic compressive stresses via the striking tools, and the junction radii are then processed to the required final contour, as a catenary shape, by a method for removing material from the surface. 
     
     
       17. The method as claimed in  claim 1 , characterized in that, in one refinement of the crankshaft segment to be processed and in the form of a catenary, the striking tools are provided with the desired catenary shape. 
     
     
       18. The method as claimed in  claim 17 , characterized in that the catenary shape of a striking tool is formed on a plane which extends in the longitudinal direction of the crankshaft, while a spherical shape is formed on a plane at right angles to the longitudinal direction. 
     
     
       19. The method as claimed in  claim 1 , characterized in that the pulsed-pressure apparatuses or striking machines are each aligned with their longitudinal axes in the striking direction, and in that the intrinsic compressive stresses are introduced by in each case only one striking tool, which is arranged in the associated pulsed-pressure apparatus or striking machine.